1. Field of the Invention
The present invention relates, in general, to a device for matching the operation of a travelling unit in a construction vehicle and, more particularly, to a structural improvement in such a matching device for reducing the required levering force of a travelling control pedal, easily controlling the output power of an engine, and safely changing the engine output power and torque in accordance with both the levering stroke of the travelling control pedal and the travelling resistance, thereby effectively controlling the travelling unit during an operation of the construction vehicle.
2. Description of the Prior Art
A device, which simultaneously controls both a main hydraulic pump and an engine of a construction vehicle, such as a power excavator, by a levering stroke of a travelling control pedal, is known to those skilled in the art.
FIG. 1 shows the construction of the hydraulic system of a typical device for matching the travelling unit in a construction vehicle. As shown in FIG. 1, when both a travelling pedal 101 and a direction select valve 103 are handled by an operator during the operation of a construction vehicle, a pilot pressure output from a pilot pump 105 is applied to a directional control valve (travelling control valve) 109 associated with a travelling motor M through both a pressure reduction valve 107 and the direction select valve 103, thereby moving the spool of the travelling control valve 109 in either direction. Both the pressure reduction valve 107 and the direction select valve 103 are synchronized with a levering motion of the travelling pedal 101. When the spool of the travelling control valve 109 is moved in either direction by the pilot pressure, pressurized fluid output from a main hydraulic pump P is controllably fed to the travelling motor M through the valve 109, thus moving the construction vehicle to the front or back in accordance with a selected direction of the direction select valve 103.
In the above state, the levering force of the travelling pedal 101 is transmitted to a throttle lever 113 of an engine ENG through both a linkage 111 and a cable 115 thus increasing or reducing the number of revolutions of the engine ENG. The linkage 111 is coupled to the lower end of the travelling pedal 101, while the cable 115, extends from the linkage 111 to the throttle lever 113 of the engine ENG.
That is, the typical matching device controls both the travelling motor M and the engine ENG by a single levering motion of the travelling pedal 101.
However, the above matching device is problematic in that it has a complex construction, so that it is difficult to design and assemble. The complex construction increases the manufacturing cost and the complexity of the manufacturing process of the matching device. In addition, slack is generated in the linkage 111, so that it may fail to precisely control the engine ENG and travelling motor M.
In an effort to overcome the above problems, a device, which converts the levering stroke of the travelling pedal into a hydraulic force and moves the throttle lever of the engine using the hydraulic force, is known to those skilled in the art. Japanese U.M. Laid-open Publication No. Sho. 61-167440 (published on Oct. 17, 1986) and Korean U.M. Registration No. 95-06053 disclose examples of such a device.
In addition, Japanese U.M. Laid-open Publication No. Hei. 3-42855 discloses a device which has an improved construction and operational effect over those of the above Japanese and Korean devices. In the above-mentioned devices, a levering motion of a travelling pedal causes a secondary pressure, thereby directly controlling a travelling control valve, while a pump and engine are controlled by a signal operation means in response to the secondary pressure.
However, the above devices are problematic in that a slow return valve included in the devices is delicate to foreign substances, thereby often failing to return the travelling control valve to a neutral position. In the above state, the devices fail to stop the travelling unit due to a hydraulic resistance, thereby causing safety hazards even though the levering force is removed from the travelling pedal. In the above devices, the direction select valve must be returned to a neutral position when it is necessary to stop the travelling unit. That is, in order to stop the travelling unit, the direction select valve must be returned to the neutral position in order to return the pilot pressure of the travelling control valve to a return tank and to switch the travelling control valve into a neutral position. Therefore, the devices may cause safety hazards when the direction select valve is not controlled by an operator during the operation of a construction vehicle.